License agreements - Merus (Details) $ / shares in Units, $ in Thousands, € in Millions, shares in Millions		1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 23, 2017 USD ($) $ / shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)
License agreements
Long term investment					$ 131,330		$ 131,330			$ 134,356	[1]
Research and development expense					298,089	$ 201,786	601,192	$ 609,706
Unrealized (gain) loss on long term investments					(34,641)	(19,574)	(11,962)	(25,388)
Merus
License agreements
Number of independent programs | item			11
Number of preclinical discovery programs | item			2
Number of programs under the resulting products are co-funded for development | item			2
Associated future global development costs , if elected to co-develop (as a percent)			35.00%
Upfront payment under license agreement		$ 120,000
Merus | U.S.
License agreements
Profit sharing (as a percent)			50.00%
Percentage of profit (losses)			50.00%
Merus | Minimum
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus | Minimum | Non-U.S.
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus | Maximum
License agreements
Number of additional preclinical discovery programs | item			9
Royalty payments on future global net sales (as a percent)			10.00%
Percentage of additional royalties			4.00%
Percentage of reverse royalty			4.00%
Merus | Maximum | Non-U.S.
License agreements
Royalty payments on future global net sales (as a percent)			10.00%
Merus | Maximum | Development and Regulatory Milestones
License agreements
Additional milestone payments under the license agreement			$ 100,000
Merus | Maximum | Commercialization Milestones
License agreements
Additional milestone payments under the license agreement			$ 250,000
Merus
License agreements
Per share price of common stock | $ / shares	$ 24.50
Discount for lack of marketability	$ 5,600
Fair value of shares on the issuance date	72,800
Total consideration paid	80,000
Long term investment	72,800
Research and development expense	$ 7,200				2,800	1,500	5,200	2,600
Fair market value of our long term investments					$ 72,800		$ 72,800			62,100
Ownership percentage (as a percent)					14.00%		14.00%
Unrealized (gain) loss on long term investments					$ 13,500	$ (23,900)	$ 10,800	$ (22,000)
Total revenues | €									€ 13.6
Net income (loss) | €									€ (73.0)
Merus | Accrued and other liabilities
License agreements
Accrued and other liabilities					$ 2,400		$ 2,400			$ 2,100
Merus | Stock purchase agreement
License agreements
Purchase of common stock under Stock Purchase Agreement (in shares ) | shares				3.2
Purchase price of common stock				$ 80,000
Per share price | $ / shares				$ 25.00
Lock-up period	18 months
Standstill period							3 years
Percentage of total shares allowed to sell during any 12-month period (as a percent)	33.00%
Percentage of total shares allowed to sell during any three-month period (as a percent)	10.00%

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.